1. Field of the invention:
This invention relates to a semiconductor laser device, especially an interferometric semiconductor laser device, which has excellent stability in longitudinal mode characteristics at oscillation.
2. Description of the prior art:
When a semiconductor laser device is used as the light source for optical communication or optical information processing systems, it is most desirable that the device attains stabilized oscillation, unaffected by the ambient temperature or the laser light output power, or by reflected light of the laser beam reflected by an external system. When the oscillation condition of a semiconductor laser device is unstable due to changes in various factors such as those mentioned above, mode competition noise or feedback-induced noise can occur due to the mutual interaction between the longitudinal laser modes and/or the longitudinal laser modes and the external modes. Also, light transmission using optical fibers leads to modal noise, resulting in a serious lowering of the capacity of the system.
Therefore, for the purpose of stabilizing the longitudinal mode characteristics at oscillation in such conventional semiconductor laser devices, a number of proposals have been made.
First, there are laser devices with high-reflectivity mirrors for the prevention of the reentry of reflected light into the laser and for an increase in the internal optical density therein resulting in the suppression of the non-lasing mode. However, these laser devices have the disadvantage of not allowing much output light to be emitted. Second, there are distributed-feedback (DFB) laser devices and distributed Bragg reflector (DBR) laser devices that contain a grating inside the waveguide. These laser devices, with strong wave selectivity made possible by this grating, have excellent stability in the longitudinal mode even at laser light turbulence. However, the process of manufacturing these devices is a complex one, and depending on the quality of the materials therefor, these devices cannot be manufactured properly. Third, there are C.sup.3 (cleaved coupled cavity) laser devices constructed by placing two semiconductor lasers or two waveguides in a line at the facets thereof. These laser devices are designed to give stability by the optical coupling of the two lasers in the longitudinal mode. However, the drawbacks of these devices are that it is difficult to dispose two laser devices so as to achieve good optical coupling, and that in order to bring about stability in the longitudinal mode over a wide range, by controlling individually the injection of carriers into the two laser devices, a high level of technical skill is required. Fourth, there are interferometric laser devices in which, by the construction of one or many reflecting sections in the inside of the waveguide of one semiconductor laser device, the entire waveguide can be divided into a number of parts, and by an interference effect between the longitudinal modes in each of these parts, stability in the longitudinal modes is achieved. An interferometric laser device has been proposed by Shyh Wang, et al., in the TEEE Journal of Quantum Electronics, vol. QE-18, No. 4, on page 610, April, 1982. In this laser, if the internal reflecting sections can be easily produced, a special manufacturing process will not be necessary, and the stability of the longitudinal mode should be good.